In cell biology, the nucleus (pl. nuclei; frae Laitin nucleus or nuculeus, meanin kernel or seed) is a membrane-enclosed organelle foond in eukaryotic cells. Eukaryotes uisually hae a single nucleus, but a few cell teeps, sic as mammalian reid bluid cells, hae na nuclei, an a few ithers hae mony. Human skeletal muscle cells hae mair nor ane nucleus, as dae eukaryotes lik fungi.
Cell nuclei conteen maist o the cell's genetic material, organised as multiple lang linear DNA molecules in complex wi a lairge variety o proteins, sic as histones, tae form chromosomes. The genes within these chromosomes are the cell's nuclear genome an are structurt in sic a wey tae promote cell function. The nucleus mainteens the integrity o genes an controls the activities o the cell bi regulatin gene expression—the nucleus is, tharefore, the control centre o the cell. The main structurs makkin up the nucleus are the nuclear envelope, a dooble membrane that encloses the entire organelle an isolates its contents frae the cellular cytoplasm, an the nuclear matrix (that includes the nuclear lamina), a network within the nucleus that adds mechanical support, muckle lik the cytoskeleton, that supports the cell as a hale.
Acause the nuclear membrane is impermeable tae lairge molecules, nuclear pores are required tae regulate nuclear transport o molecules athort the envelope. The pores cross baith nuclear membranes, providin a channel throu that lairger molecules maun be actively transportit bi cairier proteins while allouin free muivement o smaw molecules an ions. Muivement o lairge molecules sic as proteins an RNA throu the pores is required for baith gene expression an the maintenance o chromosomes. Awtho the interior o the nucleus daes nae conteen ony membrane-boond sub compartments, its contents are nae uniform, an a nummer o sub-nuclear bouks exeest, made up o unique proteins, RNA molecules, an pairteecular pairts o the chromosomes. The best-kent o thir is the nucleolus, that is mainly involved in the assemmly o ribosomes. Efter bein produced in the nucleolus, ribosomes are exportit tae the cytoplasm whaur thay translate mRNA.